In order to increase capacity and performance in a wireless network, a technology has been developed to transmit radio signals in narrow beams by using directed antennas, referred to as beamforming. When high-gain narrow beams are used for transmitting data from a network node to a wireless device, the transmission is concentrated to a narrow area, i.e. beam, inside which the wireless device is deemed to be located. Thereby, the data can reach quite far from the transmitting node without generating too much interference outside the beam, which enables high-data-rate transmission coverage also to very distant wireless devices which could not realistically be achieved with normal sector-wide transmissions, due to their lower antenna gain.
The term “network node” is used herein to denote any radio communication equipment of a wireless network, such as a base station, eNodeB or the like, which is capable of wireless communication with wireless devices. Further, the term “wireless device” is used herein to denote any communication equipment that is capable of wireless communication with a wireless network. Some non-limiting examples of wireless device include mobile phone, smartphone, tablet, laptop computer and Machine-to-Machine, M2M, device. Throughout this disclosure, the term access node could be used instead of network node, and the term Use Equipment, UE, could be used instead of wireless device.
In a wireless network where carriers of relatively high frequencies are deployed for beamforming, the resulting high gain beams are typically narrow and the coverage provided is also more unstable which will likely lead to several mobility related events such as measurement reports and handovers, since the Signal to Interference and Noise Ratio, SINR, of the transmitted radio signals may fluctuate and decrease rapidly. Whenever handover is performed in such a network, for example from one network node to another or from one carrier frequency to another, a favorable beam direction needs to be found at the handover target, such as a new network node or a new carrier frequency, towards the wireless device, e.g. in order to sustain a transmission with high data rate. Furthermore, if very high-gain narrow beamforming is employed, even synchronization or exchanging some initial control-signaling messages at the handover target may require that a favorable beam direction has been selected in advance in order for the network node and the wireless device to “hear” each other sufficiently well.
In order to find a favorable beam direction at the handover target, a so-called beam sweep procedure is typically employed. A typical example of a beam sweep procedure is that the network node forms a beam containing a synchronization signal and/or beam identification signal, in each reasonably likely direction, one direction at a time, and the wireless device may measure the signal and report back to the network which beam(s) it could detect and hear well. FIG. 1 illustrates a conventional procedure of transmitting reference signals from a network node 100 in a succession of beams denoted B1-B5 where the different beams B1-B5 may have different beam identities. The reference signals are transmitted in time intervals, such as timeslots or similar, such that transmission is done in beam B1 during a first time interval T1, in beam B2 during a next time interval T2, and so forth, as indicated in the figure.
Some suitable beam reception quality metrics that could be used in this context may include the received power or SINR which is measured and reported by the wireless device. The reporting may be transmitted from the wireless device either over an already existing connection, e.g. with the old network node or old frequency band, or over a link using the newly found beam at the handover target. In inter network node handover scenarios the candidate beams included in the beam sweep may be transmitted by multiple potential target network nodes, which may also be referred to as candidate network nodes.
In case there are multiple potential candidate network nodes that should be evaluated for handover of a wireless device, a controlling entity may request the candidate network nodes to transmit certain reference signals such as beam identification signals in certain beams according to a beam sweep. The wireless device that is considered for handover is preferably informed of which reference signals to listen for and preferably also the time intervals in which the candidate beams are to be transmitted. Optionally, the wireless device may also be informed of the order in which the reference signals will be transmitted, e.g. which reference signal that will be transmitted in which time interval.
However, the beamforming and beam transmission capability of a network node is limited, e.g. due to transmit power limitations, and beam transmission resources may be scheduled or needed for other purposes as well, such as transmissions to already connected wireless devices or beam sweeps for other wireless devices which potentially will be handed over to the network node, e.g. from yet another network node. Due to any of the above circumstances, it may well happen that a candidate network node already has all its beam transmission capabilities occupied in a time interval where it is required to transmit a candidate beam. It is thus a problem that a network node may sometimes not be able to transmit a reference signal in one or more beams and time intervals as dictated by a given beam sweep.